1. Field of the Invention
The present invention relates to an image display device and, more particularly, to a method for fabricating an image display device in which the crystal structure of a semiconductor film formed on an insulating substrate is reformed with a laser beam and active elements for a drive circuit are formed in the reformed semiconductor film.
2. Description of Related Art
An active matrix display device (which is also referred to as an image display device in an active matrix drive system or simply referred to as a display device) using active elements, such as thin-film transistors, as drive elements for pixels arranged as a matrix has been used widely. Most of image display device of this type are capable of displaying a high-quality image by disposing, on an insulating substrate, a large number of pixel circuits and drive circuits composed of active elements such as thin-film transistors (TFTs) which are formed by using a silicon film as a semiconductor film. By way of example, a description will be given to a thin-film transistor as a typical example of the active element.
It has been difficult to constitute a circuit on which high-speed and high-function requirements are placed by thin-film transistors each using a non-crystalline silicon semiconductor film (an amorphous silicon semiconductor. film) that has thus far been used commonly as a semiconductor film because the performance of the thin-film transistors represented by carrier (electron or hole) mobility is limited. It is effective in implementing a thin-film transistor with high mobility required to provide a higher-quality image to preliminarily reform (crystallize) an amorphous silicon film (hereinafter also referred to as a non-crystalline silicon film) into a polysilicon film (hereinafter also referred to as a polycrystalline silicon film) and form the thin-film transistor by using the polysilicon film. For the reformation, technology which anneals the amorphous silicon film by irradiating it with a laser beam, such as an excimer laser beam, has been used.
This type of technology associated with laser annealing is described in detail in a paper such as: T.C. Angelis et al., “Effect of Excimer Laser Annealing on the Structural and Electrical Properties of Polycrystalline Silicon Thin-Film Transistor,” J. Appl. Phy., Vol. 86, pp. 4600-4606, 1999; H. Kuriyama et al., “Lateral Grain Growth of Poly-Si Films with a Specific Orientation by an Excimer Laser Annealing Method,” Jpn. J. Appl. Phy., Vol.32, pp. 6190-6195, 1993; or K. Suzuki et al, “Correlation between Power Density Fluctuation and Grain Size Distribution of Laser Annealed Poly-Crystalline Silicon,” SPIE Conference, Vol. 3618, pp. 310-319, 1999.
A method for reforming an amorphous silicon film through crystallization by using irradiation with an excimer laser beam will be described with reference to FIGS. 34A and 34B. FIGS. 34A and 34B are views illustrating a commonest method for crystallizing the amorphous silicon film by scanning with the irradiation of an excimer pulse laser beam, of which FIG. 34A shows a structure of an insulating film formed with a semiconductor layer to be irradiated and FIG. 34B shows the state of reformation under the irradiation of the laser beam. For the insulating substrate, glass or ceramic is used.
In FIGS. 34A and 34B, an amorphous silicon film AS1 deposited on an insulating substrate SUB with an underlying film (SiN or the like, not shown) interposed therebetween is irradiated with a linear excimer laser beam ELA with a width in the range of several nanometers to several hundreds of nanometers. By moving the irradiation position in one direction (x direction) as indicated by the arrow for each pulse or each several pulses, the amorphous silicon film AS1 is scanned to be annealed, whereby the amorphous silicon film AS1 over the entire insulating substrate SUB is reformed into a polysilicon film PS1. Various processes including etching, wire formation, and ion implantation are performed with respect to the polysilicon film PSl obtained as a result of reforming the amorphous silicon film ASl by this method to form a circuit having active elements, such as thin-film transistors, in individual pixel portions or drive portions. The insulating substrate is used to fabricate an image display device in an active matrix system such as a liquid crystal display device or an organic EL display device.
FIGS. 35A and 35B are a partial plan view of a portion irradiated with the laser beam and a plan view of a principal portion of a thin-film transistor for illustrating an exemplary structure thereof. As shown in FIG. 35A, numerous crystallized silicon grains (polycrystalline silicon) ranging in size from 0.05 to 0.5 μm grow uniformly across the surface of the portion irradiated with the laser beam. Most of the crystal boundaries of the individual silicon grains (i.e., silicon crystals) are closed by themselves (the crystal boundaries are present between the silicon grains which are adjacent in each direction). The portion enclosed by the box in FIG. 35A forms a transistor portion TRA composed of a semiconductor film for active elements such as individual thin-film transistors. The conventional reformation of a silicon film indicates such crystallization.
To form a pixel circuit by using the foregoing silicon film (polysilicon film PSI) resulting from the reformation, etching is performed with respect to the crystallized silicon to use a portion thereof as the transistor portion and remove an unneeded portion thereof other than the portion serving as the transistor portion TRA shown in FIG. 35A, whereby an island of the silicon film is formed as shown in FIG. 35B. A thin-film transistor is fabricated by placing a gate insulating film (not shown), a gate electrode GT, a source electrode SD1, and a drain electrode SD2 on the resulting island PSI-L.
Although the foregoing prior art technology has formed the thin-film transistor on the insulating substrate by using the polysilicon film resulting from the reformation and thereby disposed an active element with excellent operational performance such as a thin-film transistor, the carrier mobility (the electron mobility or the hole mobility which will also be referred to simply as the electron mobility) in the channel of, e.g., a thin-film transistor using the crystal of a polysilicon film is limited, as stated previously. Specifically, since the crystal boundary of each of the particulate crystals in the polysilicon film that has been crystallized by the irradiation with the excimer laser beam is closed, as shown in FIGS. 34A and 34B, the achievement of a higher carrier mobility in the channel between the source and drain electrodes is limited. In addition, the circuit density of the drive circuit has also been increased with a recent trend toward higher definition. An active element such as a thin-film transistor in such a drive circuit having an extremely high circuit density is requested to have a much higher carrier mobility.